Floppy Lipids, Not Rigid Ones, Turn Out to Be the Secret to Ultra-Slow Drug Release

For decades, drug delivery scientists have operated on a simple intuition: to keep a drug from leaking out of its carrier too quickly, make the carrier’s membrane as stiff and impermeable as possible. A new study from researchers at Boston Children’s Hospital and Harvard Medical School, published in Nature Biomedical Engineering, upends that assumption. The team, led by Yuan Wang and Daniel S. Kohane, found that liposomes built from floppy, unsaturated phospholipids—long considered leaky and unreliable—actually load far more drug, release it far more slowly, and outperform their rigid, saturated counterparts in every metric that matters for sustained local delivery.

The problem the researchers set out to solve is a stubborn one in pharmacology. Hydrophilic, or water-loving, drugs diffuse rapidly away from an injection site, making it difficult to maintain therapeutic concentrations locally without exposing the rest of the body. This is especially dangerous for drugs with narrow therapeutic indices, where the difference between an effective dose and a toxic one is small. Existing platforms—implantable devices, hydrogels, polymeric microparticles, and conventional liposomes—each carry drawbacks, from the need for surgical implantation to complex chemical modifications that complicate manufacturing.

Liposomes, spherical vesicles made of lipid bilayers enclosing aqueous compartments, have long been favored carriers because they can encapsulate water-soluble drugs and present a diffusion barrier. The standard strategy has been to stiffen their membranes with cholesterol and saturated phospholipids possessing high phase-transition temperatures, on the theory that a less fluid membrane slows drug escape. Unsaturated lipids, whose double bonds introduce kinks into the acyl chains and lower the phase-transition temperature, were assumed to produce more fluid, more permeable membranes and therefore faster release and poorer encapsulation.

To test this assumption rigorously, the team compared liposomes made from phospholipids of identical 18-carbon chain length but varying numbers of double bonds, prepared under matched conditions with 28.6 mol% cholesterol. The model drug was tetrodotoxin (TTX), an ultrapotent local anesthetic that is extremely hydrophilic—its octanol/water partition coefficient logarithm is −4.44—and notoriously difficult to encapsulate, typically achieving only about 25% efficiency with passive loading. The results were striking: liposomes made from 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), which carries two double bonds, achieved roughly 72% encapsulation efficiency, about three times higher than the saturated DSPC formulation, with drug loading also threefold greater.

Release kinetics told an even more surprising story. In the first hour after preparation, only a small fraction of TTX escaped from DOPC liposomes—11-fold less than from the saturated DSPC version—while the four-double-bond DLPC formulation suppressed initial release 14-fold. Over seven days at body temperature, DOPC liposomes released just 1.3% of their TTX payload, roughly four times less than the saturated analogue. Because smaller particles release drug faster due to their higher surface area-to-volume ratios, and the unsaturated liposomes were in fact smaller, the effect could not be explained by size. The team also confirmed that direct chemical interactions between TTX and DOPC were absent by proton nuclear magnetic resonance spectroscopy.

The mechanism, revealed through cryogenic electron microscopy and confocal laser scanning microscopy, lay in ultrastructure. As the number of double bonds increased, the liposomes spontaneously reorganized from simple single-bilayer spheres into complex multilamellar and multivesicular (ML/MV) architectures—onions of nested membranes and vesicles-within-vesicles. A quantitative membrane area fraction, measured from fluorescently stained images, correlated almost perfectly (r = −0.99) with reduced one-hour release. A dithionite-quenching assay on fluorescently labeled liposomes independently confirmed the greater lamellarity of DOPC formulations, and liposomes made with negatively charged DOPG, which repel adjacent bilayers electrostatically and remain unilamellar, released drug nearly tenfold faster despite having the same degree of unsaturation.

The benefits extended broadly across hydrophilic molecules. Compared with saturated DSPC liposomes, DOPC formulations showed 1.6- to 3.3-fold higher encapsulation and loading for metformin, moroxydine, neosaxitoxin, the STING agonist 2′3′-cGAMP, the peptides insulin and teriparatide, and the protein interleukin-2, with one-hour release reduced 5- to 17-fold and seven-day release 2- to 9-fold slower. Notably, the more hydrophilic the molecule, the greater the advantage—consistent with the idea that each additional bilayer in the diffusion path presents a formidable barrier to water-soluble drugs. In contrast, the amphiphilic anesthetic bupivacaine, which can partition into and permeate lipid membranes, showed the opposite trend, releasing faster from more unsaturated liposomes.

The in vivo proof of principle came in a rat sciatic nerve block model, a demanding test because TTX is lethal at low doses if it escapes systemically. Free TTX at 5 micrograms was uniformly fatal, and saturated-liposome formulations at 110 micrograms produced only hours of block with signs of systemic spread. The DOPC liposomes, by contrast, delivered sensory nerve blockade lasting from 6.5 days at 80 micrograms of TTX to 13 days at 260 micrograms—50 times the lethal free dose—with no fatalities and essentially no contralateral deficits indicating systemic toxicity. When bupivacaine was co-loaded to accelerate onset, block was established within 30 minutes and lasted over eight days. Fluorescence imaging showed that both liposome types persisted at the injection site for weeks, confirming that the prolonged effect stemmed from slower release rather than slower clearance, and histology revealed inflammation and myotoxicity scores comparable to or better than the commercial liposomal bupivacaine product Exparel.

The implications reach well beyond anesthesia. A simple, injectable platform that passively achieves high loading and ultra-slow release of hydrophilic small molecules, peptides, and proteins—without covalent chemistry, aptamers, or device implantation—could transform sustained delivery of cancer immunotherapies like interleukin-2 and 2′3′-cGAMP, antiviral agents, and drugs for metabolic disease. The authors note that nanoscale extruded DOPC liposomes retained their ML/MV structure and sustained release, opening a path toward systemic applications. By reframing lipid unsaturation from a liability into an asset, the study suggests that some of drug delivery’s most stubborn constraints may yield not to more elaborate engineering, but to a reconsideration of the physics already hidden inside the lipid bilayer.

Subject of Research: Liposome-based sustained local drug delivery using unsaturated phospholipids

Article Title: Ultra-slow release of hydrophilic drugs via multilamellar–multivesicular liposomes formed by unsaturated phospholipids

Article References: Wang, Y., Xue, T., Torre, M., Han, Y., Shao, R., & Kohane, D. S. (2026). Ultra-slow release of hydrophilic drugs via multilamellar–multivesicular liposomes formed by unsaturated phospholipids. Nature Biomedical Engineering. https://doi.org/10.1038/s41551-026-01793-6

Image Credits: AI Generated

DOI: 10.1038/s41551-026-01793-6

Keywords: liposomes, drug delivery, unsaturated phospholipids, DOPC, tetrodotoxin, multilamellar vesicles, local anesthesia, controlled release, hydrophilic drugs, nerve block, biomedical engineering, Nature Biomedical Engineering